The so-called Touch-Tone (trademark of American Telephone & Telegraph Company) systems that are presently in wide use for telephone "dialing" employ a three by four switching matrix for selecting a composite audio signal comprised of one of four lower band frequencies and one of three upper band frequencies coupled into the matrix. These seven frequencies are substantially sinusoidal waveforms generated by hand-wired conventional audio oscillators that are generally housed within the telephone unit. While the existing Touch-Tone systems operate exceptionally well when properly adjusted, they are subject to becoming de-tuned by shock or vibration, thereby resulting in erroneous dialing. Furthermore, because the existing Touch-Tone generators are fabricated on an assembly line and because of the continued increase in labor costs, the Touch-Tone system costs are relatively high and steadily increasing.
The tone generator of the present invention is a solid state integrated circuit device capable of being packaged in the small IC package and at a unit cost that decreases as production increases. Because the generator is a solid state device, it is rugged and stable so that the frequencies will not vary with the shock or vibration and, further, in the event of damage or destruction of the circuit, it is very easily and readily replaced in the field.